The following discussion of the background art is intended to facilitate an understanding of the present invention only. It should be appreciated that the discussion is not an acknowledgement or admission that any of the material referred to was part of the common general knowledge as at the priority date of the application.
Nuclear quadrupole resonance (NQR) and nuclear magnetic resonance (NMR) are methods widely used for the detection and investigation of various chemical compounds. These methods are also successfully used for detecting the presence of specific substances, such as explosives and narcotics.
The probe of a pulsed NQR (or NMR) detection system is a device providing interaction between the radio frequency (RF) field of a resonant RF transmitter and a particular substance that is targeted within a sample for detection of NQR (or NMR) signals generated as a result of the NQR (or NMR) phenomena, as well as interaction between the RF field response from the target substance and the receiving part of the NQR (or NMR) detector. Strong RF pulses, typically with hundreds of wafts of power are used. In practical NQR devices, when detecting specific substances (for example explosives and narcotics), the power of RF pulses can reach several kW.
FIG. 1 illustrates a conventional system for detecting NQR (or NMR) signals from a target substance. For NMR a magnet is required but this is not shown in FIG. 1. A transmitter unit 60′ and a receiver unit 50′ are connected to a probe 80′ through a duplexer and matching circuit 40′ which switches the probe 80′ between a transmit mode and a receive mode. The transmitter unit 60′ generates RF pulses and applies the pulses to the probe 80′ during a transmitting period when in the transmit mode to irradiate a sample with RF energy and excite nuclei of any target substance contained within the sample. The pulses have a frequency corresponding to the resonant frequency of the nuclei of the substance to be detected and the probe 80′ is tuned to this resonant frequency typically by a tank circuit to optimise the Q-factor of the probe for optimal detection. After the RF pulse is applied, the probe 80′ can detect the NQR (or NMR) signal. This signal is received by the receiver unit 50′ during a receiving period when in the receive mode and is processed by a control and signal-processing unit 70′, which also generates all control and RF signals.
Strong radio frequency (RF) pulses applied to the probe produce transient signals (“ringing”). This results from the accumulation of energy in the circuit of a probe after the impact of RF pulses. This remaining RF energy must be dissipated before a probe can be effectively used to receive the NQR (or NMR) signal. After the probe has rung down, the NQR (or NMR) signal from the sample can be detected.
The duration of these transient signals, which determines the length of the recovery period of a probe, can be quite considerable—from several hundred microseconds to several milliseconds. This is particularly apparent when detecting low frequency NQR samples within a high Q-factor probe coil.
NQR frequencies of many significant explosive and narcotic substances are found in the low frequency range (0.1–6 MHz) and need to be detected within timeframes of 300 μs to 1.2 ms after irradiation of a sample containing same with RF energy for determining their existence. Hence ringing can present a major problem for detecting NQR signals lying in this low frequency range. Low frequency NMR and Magnetic Resonance Imaging (MRI) are also important for biological and medical research, as well as for some other purposes, and thus ringing also presents a problem with the detection of low frequency signals in these technologies as well.
In order to overcome this problem the signal-to-noise (SNR) ratio in the probe needs to be increased. This can be achieved by using high quality probe coils having a Q-factor ranging from between several hundred to several thousand.
The time constant of a tank circuit for a probe is generally expressed by:
      τ    =          Q              π        ⁢                                  ⁢        f              ,where Q is the quality factor and f is the resonant frequency. Thus in the case of high Q (for example around 1000) of the tank circuit, the recovery period of the probe after the irradiation of the sample with the powerful RF pulse is very long.
In the art, the requirement for a long recovery period of the probe for dissipating the transient signals prior to being able to detect NQR (or NMR) signals during the receiving period results in causing a considerable decrease in the detection sensitivity. Firstly, the induced delay in switching-on the receiver system to provide for the recovery period of the probe results in a part of the useful signal energy in any responsive NQR (or NMR) signal being lost. Secondly, this delay imposes serious time limitations when using multi-pulse sequences. For example, when using the steady-state free-precession (SSFP) or spin-locking spin-echo (SLSE) type sequences, the best detection sensitivity is achieved when the pulse spacing is optimised, which is determined by the relaxation parameters for each substance. When the recovery period of the probe is long, the optimum pulse spacing cannot be achieved in most instances, and this leads to subsequent losses in the detection sensitivity. Consequently it is desirable to get the resonant probe to ring-down as soon as possible during the recovery period.
A very high Q for a tank circuit is also undesirable during the transmitting period. With a high Q, the time constant of the tank circuit can be too long and the leading edge of the pulse envelope does not have time to develop. This results in the amplitude of the RF pulse not necessarily being able to reach its maximum value in the required time. The shape of the pulse then gets distorted and becomes “triangular” which is not always desirable.
The increase in the pulse duration leads to a reduction in its spread in the frequency domain, and consequently the RF pulse bandwidth can then become narrower than the NQR (or NMR) resonance line. In this case the resonance line will not be fully excited, which will make the SNR lower.
Too higher Q also limits the effectiveness of amplitude, frequency or phase modulating the pulse. Therefore, for the efficient NQR (or NMR) signal detection in many practical applications, such as detecting the presence of specific substances, the value of the Q-factor during the transmitting period must be lower than during the receiving period.
Various techniques have been used to reduce the ring-down time of the probe and hence the recovery period. One of the more widely used is the resistive damping technique based on the use of a resistive damping element, which is electrically coupled to the probe with diodes. In the transmit mode this resistive element damps a probe during the transmitting of the RF pulse and for some time after it, keeping a low Q-factor. When the amplitude of the transient signals reaches the lower threshold voltage, a high maximum Q-factor is then provided. A disadvantage of this method is the necessity to use a very low Q-factor to achieve a rapid diminution of the transient signals. Due to losses caused by this element, additional increased pulse power is needed. In addition, due to Johnson noise, the resistive damping element in the probe can reduce the SNR.
Techniques based on Q-switched damping are also known. This method involves active damping by switching the total Q-factor from a high maximum Q-factor during the transmit mode to a low Q-factor during the ring-down period, and back to a high maximum Q-factor during the receive mode. Q-switched damping uses actively switched elements (such as transistors, actively switched diodes, triacs or thyristors). When the transistors or diodes are used, a parasitic charge may be injected into the probe via the parasitic capacitance of these elements and so cause the probe to ring anew unless appropriate precautions are taken. Other switching elements (such as a triac or thyristor) switch themselves off after a certain recovery time. They do not require a switch off control signal. Therefore no charge is injected and no new ringing appears when the damping is removed. However, these devices offer less control.
A similar effect can be achieved when using the so-called “slow” transistor. This “slow” transistor exhibits a response time between receiving a switch-off signal and actually switching off. This response time is of the order of the damped ring-down time or recovery period of the probe. However the use of such a transistor requires cascade connection of other elements, such as resistors or/and diodes, which diminish the efficiency of damping and are a source of additional noise.
Unfortunately, the use of actively switched elements for Q-switched damping of the resonance circuit is limited by the maximum voltage capacity of these elements. In practice it is very difficult to find a suitable device capable of switching more than 1000V. For many practical purposes RF voltage can exceed this value considerably, which inevitably leads to the breakdown of the actively switched element. It is possible to find some kinds of actively switched elements, which have a maximum voltage capacity higher than 1000V, however usually this voltage still is not sufficient for practical use. There are also other reasons as to why these elements do not provide fast and efficient damping of the probe, making them unsuitable for use in NQR and NMR applications.